1. Field of the Invention
The present invention relates to a photographing lens having three lens elements, and more particularly, a photographing lens having three lens elements, suitable for low-cost cameras such as a compact camera and photo film package, and also capable of having a comparatively great angle of view.
2. Description Related to the Prior Art
A lens-fitted photo film unit is known as a product pre-loaded with unexposed photo film, and usable for taking a photograph immediately after purchase. Examples of lens-fitted photo film units include a high-sensitivity type loaded with ISO 1600 photo film, and clear lens-aperture type provided with a photographing lens with an aperture of f/6.2. Also, portable electronic devices for photographing an image have been recently used more and more widely, such as a digital camera, camera-equipped cellular telephone and the like. As scenes to be photographed have become varied considerably, demand for the photographing lens having a great angle of view and a large diameter has been increased for suitability to various kinds of images which changes in the region to be photographed.
One typical example of the photographing lens having a great angle of view and large diameter is a triplet type, which is a single-focus lens including three lens elements being positive, negative and positive in a sequence from an object side to an image side (image surface side). JP-A 7-035972 discloses an example of the triplet type, in which two positive lens elements are formed from high refractive index material of low dispersion. The one negative lens element at the center is formed from low refractive index material of high dispersion. This combination is for the purpose of compensating for curvature of the field in the performance of the photographing lens as optical system. However, there is a shortcoming in that the main use of the high refractive index material is likely to raise the manufacturing cost.
U.S. Pat. No. 5,606,461 (corresponding to JP-A 8-201686) and U.S. Pat. No. 5,754,347 (corresponding to JP-A 9-133860) disclose the photographing lens with a reduced cost owing to the use of material with a relatively low refractive index. In the embodiments Nos. 1–6 of U.S. Pat. No. 5,606,461 (corresponding to JP-A 8-201686), the photographing lens has an aperture of f/4 as a highly clear aperture lens, but has a shortcoming in a limit in the angle of view as small as 70° or less. In the embodiments Nos. 7–11 of this patent, the photographing lens has the angle of view as great as 72°, but has a shortcoming in a small amount of light, namely the lens aperture as small as f/4.5. U.S. Pat. No. 5,754,347 (corresponding to JP-A 9-133860) discloses the photographing lens in which a lens element is formed from material with a lower refractive index than that according to U.S. Pat. No. 5,606,461. However, the photographing lens according to this has shortcomings in smallness in the angle of view, and a small amount of light, namely the lens aperture as small as f/4.5 (See the fifth embodiment of the document).
There are other three-element lenses different from triplet types, for example, U.S. Pat. No. 6,775,072 (corresponding to JP-A 2002-350720). JP-A 8-220430 discloses the photographing lens having the lens elements which are negative, positive and positive in a sequence from the object side. However, a problem of this disclosure lies in considerable distortion as great as 15.9%. JP-A 2003-149545 discloses the photographing lens having the lens elements which are negative, positive and negative in a sequence from the object side, and has f/2.8 as a sufficiently clear aperture lens. However, a problem of this disclosure lies in an upper limit of the angle of view as small as 65°.
There is no known techniques of the photographing lens in which the greatness in the angle of view is obtainable at the same time as a low cost and a sufficiently clear lens aperture.